Polyacetal is a high-crystalline engineering plastic and is widely used as a gear such as spur, helical, screw, pinion and rack gears for structural parts and the like of precision apparatuses in the electric and electronic field because it is excellent particularly in slidability and fatigue resistance in addition to mechanical characteristics. Polyacetal also features excellence in moldability, which often allows these gears to be produced by an injection molding method.
The quality and performance of products have been improved in the field of precision apparatuses in recent years; as a result, there is also a need for improved performance for the gears used. By way of example, an image-forming apparatus such as a laser printer, a facsimile machine and a copier is sturdier, smaller and capable of splendid printing with more rapidity and precision, such that it has a higher commercial value. In the image-forming apparatus, gears are used as parts for transmitting and controlling the movement, angle, etc. of a photoreceptor drum, a developing roller or the like. Thus, these gears are being required not only to have improved durability and dimensional accuracy compared to conventional gears but also to be particularly excellent in rotation transmission accuracy.
Here, the dimensional accuracy of gears refers to roundness and pitch error, tooth profile error, tooth trace error, tooth space runout, etc. Here, the roundness indicates whether the initial gear shape forms a proper circle; the pitch error, tooth profile error, tooth trace error, tooth space runout, etc. represent accuracy against the initial specifications (the initial gear shape) and are values measured by the method prescribed in JIS D 1702. The rotation transmission accuracy is the accuracy of rotation transmission when the gear is actually driven; specifically, it is represented, for example, by the double or single flank meshing error prescribed in JGMA (Japan Gear Manufacturers Association) 116-2.
The dimensional accuracy sometimes suggests a relation with the rotation transmission accuracy. However, a gear practically rotates in a state subjected to torque; therefore, thrust is produced on the flank, which may result in the gear moving with deformation or deflection. Thus, a gear excellent in dimensional accuracy sometimes cannot necessarily be said to be excellent also in rotation transmission accuracy. In addition, a mechanism is present in which the torque loading on a gear varies during the rotation thereof; it may also be necessary that the rotation transmission accuracy does not change.
As conventional techniques, methods for improving the dimensional accuracy are disclosed which include, for example, a method involving improving a decrease in the module, an arrangement of a gear shape such as thinning and the design of a rib or the like as a reinforcing mechanism (e.g., patent documents 1 and 2), a method involving performing improvement in the method of producing the gear, such as partially pressuring the molded product, or using the reduction of resin viscosity by allowing the molten resin to contain a gas (e.g., patent documents 3 to 6), a method involving subjecting the prepared gear to secondary processing such as aging treatment, and, further, a method involving achieving accuracy by designing a gear train (e.g., patent document 7). The above techniques allow gears to have sufficient durability and are also effective for dimensional accuracy. However, a problem has been posed that the type, rib structure and the like of the gear used are limited, and a problem has further been present that equipment investment is required for producing a gear, e.g., it is necessary to modify an injection molding machine.
Accordingly, methods have been attempted for improving materials themselves used as gears to provide gears excellent in dimensional accuracy and rotation transmission accuracy. Examples thereof include a method involving optimizing the proportion of a copolymerized component in a polyacetal (e.g., patent document 8), a method involving blending a particular ester compound in a polyacetal (e.g., patent document 9) and, further, a method involving blending wollastonite having a particular shape and a sliding agent in a polyacetal (e.g., patent document 10). However, the above methods for improving materials have some effect, but have yet failed to be sufficiently satisfactory in rotation transmission accuracy.
Reinforcement by adding a filler seems to be effective to obtain a gear excellent in dimensional accuracy and rotation transmission accuracy by improvement of a material. However, the method involving adding a filler is known to reduce the slidability of a polyacetal and tends to be poorly used for gears. (For example, non-patent document 1). In addition, the addition of a filler is described to be hardly capable of having a sufficiently satisfactory effect on the accuracy of gears. (For example, patent documents 6 and 8).
Thus, it has been difficult in conventional techniques to obtain a durable gear well-balancedly having the dimensional accuracy and rotation transmission accuracy being required in recent years by an injection molding method, which is excellent in industrial productivity.
Patent Document 1: JP-A-11-013861
Patent Document 2: JP-A-11-082686
Patent Document 3: JP-A-2001-323990
Patent Document 4: JP-A-2001-323991
Patent Document 5: JP-A-2002-096366
Patent Document 6: JP-A-2002-031213
Patent Document 7: JP-A-2004-340160
Patent Document 8: JP-A-11-051154
Patent Document 9: JP-A-2002-020577
Patent Document 10: International Publication WO9943751 (Corresponding U.S. Pat. No. 6,391,956)
Non-Patent Document 1: Molded Plastic Gears Handbook, p 58-68, 1st Edition, edited by Molded Plastic Gear Research Expert Committee of Precision Engineering Institution, published by Sigma on Apr. 20, 1995.
The present invention is a gear made of a polyacetal resin composition comprising a polyacetal and a filler having a particular shape and particle diameter. An object thereof is to provide a gear which has durability and is excellent in the balance between dimensional accuracy and rotation transmission accuracy.